The disclosed invention is concerned generally with methods for measuring the output reflection coefficient or return loss of circuits and more particularly with measuring the output reflection coefficient of an automatic level controlled source (i.e., an ALC source). An ALC source (also known as a level compensator) is a circuit which produces an output signal having a precisely regulated amplitude.
In a typical ALC source shown in FIG. 1, a source signal is applied to the input of a modulator to produce an output signal. An amplitude detector is employed to produce a d.c. signal proportional to the amplitude of the amplifier output signal. The difference between the d.c. signal from the amplitude detector and a reference signal V.sub.ref is produced and is used to regulate the gain of the modulator in order to hold constant the amplitude of the output signal. The value of the amplitude is determined by the value of V.sub.ref.
After the leveling loop has settled to its steady state condition, the modulator holds the amplitude of the output signal nearly constant, so that the output of the modulator is electrically equivalent to an ideal voltage generator. The output impedance of the ALC source is selected by attaching an impedance element of selected impedance in series with the output of the modulator. The value of output impedance is typically selected as a standardized value of 50 ohms or 75 ohms.
ALC sources are often employed at the output port of signal generators to precisely define the amplitude of generated signals. The output reflection coefficient of the signal generator affects the responses of test circuits to signals from the signal generator so that it is important to precisely determine the output reflection coefficient of the signal generator. This determination in turn requires the ability to measure the output reflection coefficient of an ALC source. Although the output impedance is typically selected to be 50 ohms or 75 ohms, stray capacitance and inductance can produce parasitic impedances which contribute to the output impedance. In addition, errors can be made in the selection of the impedance element which defines the output impedance of the ALC source so that it is important to be able to test the actual output impedance of an ALC source and not just calculate it from the impedances of its component elements. The parasitic impedances are typically voltage dependent and thus must be measured under standard operating conditions. Also, the output of the amplifier will only approximate an ideal voltage source when the ALC source is active so that a method must be devised for measuring the output reflection coefficient of an ALC source under operating conditions.
It is well known that the output reflection coefficient of a circuit can be measured by attaching a directional bridge to the output port of the circuit and launching a test signal through the bridge towards the ALC source. Part of the test signal reflects at the output port of the ALC source due to the inequality between the circuit output impedance Z.sub.out and the characteristic impedance Z.sub.o of the transmission line. The ratio between the reflected signal and the test signal equals the reflection coefficient p which is related to output impedance and the characteristic impedance by the relation p equals (Z.sub.out -Z.sub.o)/(Z.sub.out +Z.sub.o) (see Millman and Taub, Pulse, Digital, and Switching Waveforms, McGraw Hill Book Co., 1965, pages 90-93). The output impedance can thus be determined from Z.sub.o and from the relative amplitude and phase between the test signal and the reflected signal.
Surprisingly, this method for measuring output impedance fails when applied to an ALC source. It was expected that the test signal would produce a simple reflected signal to add to the generated signal of the ALC source. However, when the frequency of the test signal is within the bandwidth of the leveling loop, a pair of additional signals are produced which are proportional to the amplitude of the test signal but are not proportional to the reflection coefficient. The prior art methods for determining output impedance are thus inapplicable to ALC sources.